Psychophysics Flashcards
What is psychophysics
Quantitative branch of the study of perception
What does psychophysics examine
The relations between observed stimuli and responses, as well as the reasons for those relations
Who founded psychophysics
Gustav Fechner
When was psychophysics founded
October 22, 1850
What was Gustav Fechner seeking to do when founding psychophysics
Wanted to calibrate the mind relative to physical stimuli
What is the first requirement in a psychophysics experiment
Human/nonhuman subjects are tested in an expirimental environment that maximizes control of stimulus variations over variations in the subjects responses
What is the second requirement of psychophysics experiments
Stimuli carefully controlled, often varying along only a single physical dimension (ex: intensity)
What is the third requirement for psychophysics experiments
Subject’s responses are highly contrained (ex: “yes, i see stimulus” or “no i dont see stimulus”)
What is the fourth requirement of psychophysics experiments
Small numbers of subjects are tested with extensive within-subject designs
What do psychometric functions typically look like
Sigmoid “S” shape with the x variable being invisible near zero and visible near 100, and the y variable being percent that said “yes”
What are the three types of measurements in a psychophysics experiment
Absolute threshold
Difference threshold
Point of subjective equality (PSE)
What is the absolute threshold
Lower limit of perception; weakest stimulus that can just barely be detected
What is the difference threshold
Smallest reliably discriminable difference between two stimuli
What is difference threshold also known as
JND (just noticeable difference)
What is the point of subjective equality (PSE)
Magnitude of one stimulus at which it is perceived as equivalent in magnitude to another
What percentage of people are able to notice the PSE
50
What percentage of people are able to notice the JND
75
What is the x axis gap between the PSE and JND known as
IND
What are the two techniques for measuring thresholds and PSEs
Yes-No Tasks
X AFC Tasks
What are Yes-No Tasks
NO option is the absence of something; typically used in detection tasks
Are Yes-No Tasks subject to bias?
Yes. Bias is corrected by the SDT
What happens in Yes-No Tasks when the participant is unsure
Answer is guided by personal biases
How is the threshold determined in Yes-No Tasks
Threshold set to 50%
What are X AFC Tasks
A choice between multiple, corresponding alternatives; used in discrimination tasks
Are X AFC Tasks subject to bias
No
What happens when subjects in X AFC Tasks are unsure
They’re forced to guess
What is the “pure” guessing performance determined by in X AFC (with example)
X (example, for 2 AFC = 50%, 4 AFC =25%)
What does the guessing range determine
The lower performance limit
What is the threshold set to in X AFC tasks
Set to halfway between the guessing rate and 100%
What are the three procedures for measuring thresholds and PSEs
Method of adjustment/limits
Method of constant stimuli
Staircase method
What is the method of adjustment/limits
Person adjusts stimulus to value where it is barely detectable (absolute threshold) or adjusts one stimulus until it matches another (PSE) or differs from another (JND)
What is the difference between the method of adjustment and the method of limits
Adjustment: Stimulus adjusted by subject
Limits: Stimulus is manipulated by the experimenter or a computer program
How are the results of the method of adjustment/limits measured
Lower/raise until just barely detectable
Repeat X number of times
Average the values to estimate the threshold
What is the method of constant stimuli
Present stimuli at various levels; person responds (ex: yes or no) following each presentation, generating a psychometric function
Describe method of constant stimuli in depth
Display stimuli in random order and repeat many times
For each trial, subject indicates their perception of the stimuli
Plot subjects’ answers as response probablility vs stimulus value
Estimate the threshold and/or slope
How many times should the constant stimuli method be repeated per stimulus level
~40
What is the staircase method
Adaptive search for threshold
What does a subject’s response (correct/incorrect) determine in the staircase method
Correct -> following trial is harder
Incorrect -> following trial is easier
What are the two types of staircase method
Fixed step size
Adaptive step size (asking where somebody is from analogy)
What is the staircase method designed to do
Converge to a certain threshold value
What are the pros of the constant stimuli method
Gold standard of psychophysics
Yields a complete psychometric function
What are the cons of the constant stimuli method
Takes many trials
Requires pre-selecting stimulus levels
What are the pros of the staircase method
Fastest method for estimating threshold
Doesnt require pre-selecting stimulus levels, just the starting point
Great for non-expert subjects
What are the cons of the staircase method
Does not yield a complete psychometric function
Focuses on estimating the threshold
